1. Field of the Invention
The present invention generally relates to cutoff frequency adjusting methods, GmC filter circuits and semiconductor devices, and more particularly to a cutoff frequency adjusting method for adjusting a cutoff frequency of a GmC filter, a GmC filter circuit which employs such a cutoff frequency adjusting method, and a semiconductor device having such a GmC filter circuit.
2. Description of the Related Art
Electronic apparatuses having an analog data processing circuit, such as portable telephones, includes a filter circuit for noise reduction and the like. A cutoff frequency of the filter circuit deviates depending on inconsistencies that are introduced at a production stage of elements forming the filter circuit, and for this reason, the filter circuit in most cases cannot eliminate all target frequencies.
FIG. 1 is a circuit diagram showing an example of a conventional GmC filter circuit. A GmC filter circuit 1 includes an Operational Transconductance Amplifier (OTA) circuit 2 and a capacitor C that are connected as shown in FIG. 1. If an input signal to the GmC filter circuit 1 is denoted by Vin, an output signal of the GmC filter circuit 1 is denoted by Vout, a capacitance of the capacitor C is denoted by C, and a gm value of the OTA circuit 2 is denoted by Gm, the output signal Vout can be represented by Vout=(Gm·Vin)/jωC. A cutoff frequency Fc of the GmC filter circuit 1 is determined by Fc=Gm·Vin/C. A GmC value of the GmC filter circuit 1 can be set arbitrarily by setting the Gm value of the OTA circuit 2 and the capacitance C of the capacitor C to appropriate designed values.
The GmC filter circuit 1 has a Low-Pass Filter (LPF) characteristic. However, when forming the GmC filter circuit 1 in a semiconductor integrated circuit such as a Large Scale Integrated (LSI) circuit, the Gm value and the capacitance C deviate from the respective designed values due to inconsistencies introduced at the production stage of the semiconductor integrated circuit, and the desired LPF characteristic cannot be obtained. In other words, even if the Gm value and the capacitance C deviate from the respective designed values, the cutoff frequency Fc of the GmC filter circuit 1 cannot be determined because there is no means for detecting the GmC value, and it is difficult to form the GmC filter circuit 1 which can maintain the cutoff frequency Fc to a constant value.
In addition, according to another example of the conventional GmC filter circuit, a capacitor having a capacitance equal to or proportional to the capacitance of the capacitor within the GmC filter circuit is provided within the OTA circuit in order to maintain the GmC value constant. In this case, a switched capacitor may be used for the capacitor within the OTA circuit, and the GmC value can be controlled constant by constantly inputting an arbitrary clock to the GmC filter circuit. However, in this GmC filter circuit which maintains the GmC value constant, the clock must constantly be input to the GmC filter circuit, and it is difficult to reduce the power consumption of the GmC filter circuit. For this reason, this kind of GmC filter circuit is unsuited for use in electronic apparatuses which require low power consumption, such as portable telephones.
For example, a filter circuit using a conductance amplifier (Gm amplifier) is proposed in a Japanese Laid-Open Patent Application No. 2004-266316. GmC filter circuits are proposed in Japanese Laid-Open Patent Applications No. 2003-8398, No. 2005-348109 and No. 2005-286778, for example. The Japanese Laid-Open Patent Application No. 2005-348109 proposes a GmC filter circuit using a switched capacitor.
Therefore, in the conventional GmC filter circuits, it was conventionally difficult to simultaneously maintain the GmC value constant and realize a low power consumption.